Institut für Astronomie und Astrophysik

Abteilung Astronomie

Preprint M/98

LOW LUMINOSITY STATES OF THE BLACK HOLE CANDIDATE GX 339-4. II. TIMING ANALYSIS

MICHAEL A. NOWAK (1) , JÖRN WILMS (2) , JAMES B. DOVE (1);(3)

(1) JILA, University of Colorado, Campus Box 440, Boulder, CO 80309­0440, USA
(2) Institut für Astronomie und Astrophysik, Astronomie, Universität Tübingen, Waldhäuser Str. 64, D-72076 Tübingen, Germany
(3) CASA, University of Colorado, Campus Box 389, Boulder, CO 80309­0389, USA

1999, ApJ, 517, 355-366

Abstract. Here we present timing analysis of a set of eight Rossi X­ray Timing Explorer (RXTE) observations of the black hole candidate GX 339-4 that were taken during its hard/low state. On long time scales, the RXTE All Sky Monitor data reveal evidence of a 240 day periodicity, comparable to timescales expected from warped, precessing accretion disks. On short timescales all observations save one show evidence of a persistent f(QPO) of about 0.3 Hz QPO. The broad band (10^-3 - 10^2 Hz) power appears to be dominated by two independent processes that can be modeled as very broad Lorentzians with Q less than or about 1. The coherence function between soft and hard photon variability shows that if these are truly independent processes, then they are individually coherent, but they are incoherent with one another. This is evidenced by the fact that the coherence function between the hard and soft variability is near unity between 5x10^-3 - 10 Hz but shows evidence of a dip at f about 1 Hz. This is the region of overlap between the broad Lorentzian fits to the PSD. Similar to Cyg X-1, the coherence also drops dramatically at frequencies greater than or about 10 Hz. Also similar to Cyg X-1, the hard photon variability is seen to lag the soft photon variability with the lag time increasing with decreasing Fourier frequency. The magnitude of this time lag appears to be positively correlated with the flux of GX 339-4. We discuss all of these observations in light of current theoretical models of both black hole spectra and temporal variability.

Key words: accretion - black hole physics - Stars: binaries - X­rays: Stars

Paper (101k gzip'ed Postscript including figures)

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Jürgen Barnstedt (barnstedt AT astro.uni-tuebingen.de) Last updated 14-Dec-1998